curry.js

module.exports = curry;

function curry (f) {
	var arity = f.length;
	var params = [];
	var end = createEnd(f, arity);
	return createCurried(params, arity, end);
}

function createEnd (f, arity) {
	var src = 'return function ' + f.name + '_curried (args) { return f(';
	for (var i = 0; i < arity; i++) {
		src += (i ? ', args[' : 'args[') + i + ']';
	}
	src += '); };';
	return (new Function ('f', src)(f));
}

function createCurried (collected, arity, end) {
	return function () {
		var params = concat(collected, arguments);
		return params.length < arity
			? createCurried(params, arity, end)
			: end(params);
	};
}

function concat(a, b) {
	var al = a.length;
	var bl = b.length;
	var c = new Array(al + bl);
	var i;

	for(i=0; i<al; ++i) {
		c[i] = a[i];
	}

	for(i=0; i<bl; ++i) {
		c[i+al] = b[i];
	}

	return c;
}

function add(x, y) {
	return x + y;
}

var addc = curry(add);

var z = addc(1)(2);
console.log(z);

This variant of @unscriptable's original version uses an extra dynamic compilation wrapper (line 16), to allow naming the last function in the curry chain, which could make debugging easier. It probably has similar perf characteristics.

Update: I added a hand-coded concat() for appending arguments. It's significantly faster than using Array.prototype.concat + slice.